The massive Tappan Zee Bridge spans the Hudson River just north of New York City. For Alex Morell, it's a bridge of tears because of what happened there to her sister, Annie Morell Petrillo, 38, last September.

We first met Alex and Annie back in 1994. The look-alike sisters had distinct personalities; Alex outgoing, Annie more sensitive and shy. But, Alex said, they were inseparable. "It was never Annie or just Alex," she said. "It was always Alex and Annie."

The sisters had a carefree childhood, growing up in the wealthy New York suburb of Bronxville. Their mother, Anne, an heiress to the Scripps newspaper fortune, was just a regular mom. "She didn't think of herself as having a lot of money," Annie said back in 1994. "She just thought of herself as a normal person."

But when the sisters were teenagers, their parents divorced. Anne Scripps Morell was left vulnerable, hurt and lonely. Soon, however, she seemed to find happiness again in the arms of Scott Douglas, a handsome man nine years her junior, who described himself as a successful contractor. The two were married after less than a year.

"We were surprised," Annie said in 1994. "We couldn't believe it. But we were also happy because my mom, we thought, was happy."

Alex, however, was more suspicious of their new stepfather. Over time, it became clear her suspicions were justified. Rather than a successful contractor, Scott Douglas was more like a struggling house painter. He expected Anne to support him in style, her daughters said.

"He would scream at her to buy him cars; that he wants new clothes, new shoes, new everything," Annie told ABC. "He said he deserved it."

The fights got worse and worse but Anne, her daughters believe, didn't want to admit another failed marriage to her friends. Instead of a divorce, she and Scott had a baby, Victoria, called Tori, in June 1990.

"I think my mother thought that once Victoria was born that everything, the violence, would just end," Alex. said.

It didn't. Anne told her daughters that Douglas threw her down the stairs, and even tried to push her out of a car on the highway. He would threaten to take Victoria if Anne tried to get a divorce. It got so bad, the sisters said, that Anne sometimes slept in her daughter Annie's room with a drawer blocking the door -- and a hammer by the bed for protection.

Finally, Anne decided on divorce. A family court judge granted an order of protection but Douglas was allowed to remain in the house. In December 1993, Anne called police three times to complain about Douglas but, with no evidence of violence, he wasn't arrested.

In late December, Anne returned to court, hoping to force Douglas out of the house. But with the judge on vacation, she was told to come back after New Year's.

'I Got Very Scared'

Then came New Year's Eve. Alex was going away on a ski trip and Annie was planning to go to a party. Earlier in the day, Anne and Scott had had another fight. "She kept crying, so I said I'm not going to go out," Annie said.

But her mother insisted they both go out and have fun.

When Annie got home at 3:30 that morning, the door was locked. "I was pounding on the door," Annie told ABC in 1994. "And then I noticed his car was gone, so I got very scared."

When police and firemen arrived, they broke down the door and found Anne Scripps Douglas in Annie's bed, bleeding from the head. The wounds were so severe, it was at first thought she had been shot. It was later determined she had been attacked with a hammer, apparently the same one she slept with for protection.

Alex said she immediately suspected Scott had done it. "I knew it from the second they told me that it was him," she said.

Police found Douglas' grey BMW abandoned on the Tappan Zee Bridge, the engine running, a bloody hammer on the front seat. They suspected he had jumped, but no one was sure.

But Annie and Alex were sure -- that Scott had faked his own suicide. They thought he was on the run, and might be coming after them next.

Anne Scripps Douglas died six days later without ever regaining consciousness. Annie and Alex were heartbroken.

"My mother was our best friend," Alex told ABC in 1994. "It's so hard when you lose your mother and your best friend."

The horror was compounded by the realization that their stepfather was the suspect. Their 3-year-old sister, Victoria, had apparently witnessed the gory scene. "She hid under the bed from daddy when he was calling her name, and she said, 'Why did daddy hurt mommy?'" Annie said in 1994. "'Why does mommy look like a monster? Why does mommy have paint all over her face?'"

Alex and Annie had no doubt that Douglas was still at large. "I don't see Scott jumping," Annie said. "I don't think he could take his own life. I think he was a coward."

The Scripps family even offered a $100,000 reward for Douglas' capture. But three months later, the winter ice gave up Scott Douglas' body. He had indeed jumped from the Tappan Zee Bridge sometime around midnight on New Year's Eve. Annie and Alex no longer had to fear for their safety but closure would not come so easily.

"I felt guilty," Annie told ABC in 1994. "I just felt it was my fault at first, too, that I shouldn't have gone out, and that I could have stopped it. Our lives will never be the same again."

Over the years, Annie and big sister Alex tried to get on with their lives. They got married, had children and got divorced. Through the ups and downs, the sisters stayed extraordinarily close.

"We'd talk probably like 10, 15 times a day," Alex told "20/20"'s Deborah Roberts. "We went on vacation together, and if I went on vacation, I called her 10 times a day, and she would call me. And all my friends were like, 'Oh, my gosh, you girls are nuts.'"

'I Miss Mommy, I Want Mommy Back'

For Annie, so close to and so like the vulnerable mother for whom she was named, getting on with life was more of a struggle.
"She felt so guilty because she said if she [had] stayed home, she thinks she would have prevented it," Alex said of her sister. "I said, 'No, Annie.' I said 'No, he would have killed you, too.'"

Alex says her sister saw a therapist but to no avail. "She would break down into tears, saying, 'I miss mommy, I miss mommy, I want mommy back' ... almost like she was a little kid again."

Harvard professor Holly Prigerson said Annie had classic symptoms of a condition known as "prolonged grief disorder." Grief for a loved one can go on too long and become pathological, Prigerson said.

The risk factor is the attachment between the survivor and the deceased. "It's not how the person died," Prigerson said. "It's that sense of attachment and dependency; that I need this person to feel safe, secure, happy, whole."

Annie's friends said she talked about how much she missed her mother every day. But they had no idea how serious her mental suffering was.

"Hanging out with her, and being with her, you would never think that she was depressed," friend Rosey Kalayjian said. "She loved to laugh."

Kalayjian and other friends believe it was at night, when she was alone, that Annie's troubles would come back. "She would start thinking about everything again," friend Hillary Karmilowicz said. "And a lot of the images that she saw still came back to her."

Annie's friends said she had other worries, too -- problems with money and love. "I always say Annie, well, was like a little tortured soul," Kalayjian said.

"No matter how many flowers you buy her, no matter how many presents or fun nights out, deep down inside there is something nobody can fix."

As if memories weren't enough, there was also a TV movie of her mother's story, "Our Mother's Murder," continually reminding Annie of the past. "She used to get so depressed when it was on," sister Alex said. "So depressed, and she'd break down and go into tears and she'd turn it right off."

The movie aired again in mid-September. Alex said Annie mentioned that "mom's movie" was on again, but she's not sure if Annie watched it.

Two days later -- Sept. 24, 2009 -- seemed to start like any other day. After dropping her son off at school, Annie ran into a friend, beeping and waving as usual.

Then she met Alex. The two had lunch, followed by a pedicure and manicure. Then the two hung out at Annie's house until 6 p.m.
"We were just laughing, you know -- talking about I have no clue what, but she was fine," Alex said.

About the time Alex was leaving, Annie sent friend Kalayjian a cheerful text message. Shortly after that, she got into her car and drove to the Tappan Zee Bridge. Police say Annie stopped her car in the middle of the bridge. She got out smoking a cigarette, and, at 7:50 p.m., she jumped. Her body was found in the river three days later.

Why the Same Way, the Same Place?

Police found a pencil-written suicide note in her car. It begins with a goodbye to her 13-year-old son, Michael.

"My little Michael, my angel -- I loved you more than life," the note read. "I will love you forever. I tried to give you everything and will always be there. I'm so sorry I let you down. ... Alex and Alexa [Alex's daughter], I love you so much. Thank you for the smiles and for never giving up. ... Mommy and Daddy -- please find me!"

Alex said she cannot fathom what her sister was thinking when she jumped, especially after the "beautiful, fun day" the sisters spent together. "I have no clue what went through her head," Alex said. "No clue, and it's racking my mind."

Annie had made past suicide attempts, according to police records. But the haunting question remains: Why this way? Why the same way, the same place, as her stepfather so many years ago?

"I don't know," Alex said. "That's another thing that blows my mind."

"The only thing I can think of," friend Karmilowicz said, "was that maybe she didn't want Alex to have to find her, the way she found her mom."

Another friend offered a different theory. "Maybe it's her way of saying that her life essentially stopped" the night Scott Douglas killed her mother, professor Prigerson said. "It definitely wasn't a coincidence that she jumped from the same bridge that her stepfather did."

Karmilowicz said it was important for people to realize what a vicious cycle domestic abuse is. Annie lost her mother, and now Michael has lost his.

Alex said she was angry at her sister "for leaving behind her son, myself, her friends. ... I think that was the most selfish thing to do, and it breaks my heart that I didn't know she was in so much pain."

Now it is Alex who is overwhelmed by grief.
"This is 10 times tougher than my mother's murder," she said. "Ten times, it hurts so much. ... I just want her back. It just kills me because she was my best friend. I always feel so alone now."

Experts say there are effective treatments for depression and complicated grief like Annie's. Alex, despite her grief, is determined to move on.

"My mother never would want my sister and I to live in the past and harbor the fact that she was murdered," Alex said. "I think she would want us to move on in life, and I think Annie would want that, too."